This invention relates to a method of producing a solid electrolyte capacitor which has a layer of manganeses dioxide as the solid eletrolyte, and more particularly to a method of forming a manganese dioxide layer of improved quality on the surface of a dielectric oxide coating formed on the anode.
Manganese dioxide is known and widely used as an excellent electrolyte material in a solid electrolyte capacitor. In conventional methods for the fabrication of a solid electrolyte capacitor having an anode of a valve-metal typified by aluminum and tantalum and a thin layer of manganese dioxide as the electrolyte, the manganese dioxide layer is formed by initially anodizing the valve-metal to form thereon a thin oxide coating which serves as the dielectric of the capacitor, immersing the oxide-coated anode in an aqueous solution of manganese nitrate, and then heating the wet anode to about 130.degree.-135.degree. C. in a hot-air conveying or circulating furnace to cause pyrolytic decomposition of the adhering manganese nitrate to manganese dioxide. It is very difficult, however, to form a practically serviceable manganese dioxide layer uniformly over the entire area of the dielectric layer by a single cycle of these procedures. It is necessary to repeat about 10 times a series of procedures consisting of immersion in the manganese nitrate solution, heating for the pyrolytic decomposition and then reforming of the dielectric coating. Throughout the repeated heating, the pyrolytic decomposition of manganese nitrate occurs in a stream of hot air practically under atmospheric pressure and heat for the decomposition is transferred to the wet anode mainly by convection.
A manganese dioxide solid electrolyte capacitor produced by performing the pyrolytic decomposition in the foregoing manner has a capacitance very close or approximately equal to a theoretical value, but involves several shortcomings such as a comparatively large loss value and leakage current, a relatively poor insulation or breakdown voltage and/or an unsatisfactory frequency dependence of the capacitance. These shortcomings are considered to be greatly attributable to physical properties of the manganese dioxide layer of the capacitor. A manganese dioxide layer formed by a conventional pyrolysis process consists of relatively large-sized and irregularly shaped manganese dioxide particles and is very porous, relatively low in density and weak in adhesion strength to the dielectric coating. As an additional disadvantage of such a manganese dioxide layer, the layer tends to have an extremely rugged surface. Sometimes the surface includes local elevations of considerable heights and widths, so that an extra machinning work for removing such elevations is needed in advance of forming a counter electrode on the manganese dioxide layer to encase the produced capacitor element in a predetermined case. The machinning work is not only inconvenient to industrial production of the capacitor but also disadvantageous for the quality of the capacitor since this work is liable to damage the dielectric coating and cause an increase in the leakage current.
An improved method for the pyrolytic decomposition of manganese nitrate was proposed recently to obviate the above described disadvantages of a manganese dioxide layer formed by a conventional method. (Among the inventors of the present invention, Nishino and Hayakawa participated in the studies which brought about this improved method.) In this method, the pyrolytic decomposition is carried out in a semi-closed heating chamber of a radiant furnace so that the decomposition may proceed under a positive pressure of a small magnitude attributable to prolonged retention of gaseous decomposition products in the heating chamber. A manganese dioxide layer formed by this method is very smooth and dense due to uniformly distributed and relatively small particle size of the manganese dioxide particles. A solid electrolyte capacitor including this manganese dioxide layer exhibits smaller loss value and leakage current than a similar capacitor produced by the use of a hot-air circulation furnace in forming the manganese dioxide layer.
The capacitor including a manganese dioxide layer formed in a semi-closed radiant furnace, however, has an unfavorable tendency that the capacitance as a relative value to a theoretical value is slightly smaller than the same value for a similar capacitor produced by a conventional method. Additionally, a manganese dioxide layer formed by the improved method is sometimes not satisfactorily resistant to a mechanical stress particularly when the manganese dioxide particles are extremely small-sized and uniformly distributed. This tendency is disadvantageous particularly for producing a relatively large-sized capacitor since the manganese dioxide layer in such a case is liable to be subjected to various stresses during subsequent procedures for providing a counter electrode and a protective cover and the capacitor exhibits an increase in the leakage current.